The present invention relates in general to driver circuits for controlling the current flowing through an element while receiving a data signal, and more specifically to a laser diode driver circuit that allows for high-speed switching.
FIG. 1 shows a prior art driver circuit (100) used in optical communication and other fields. The driver circuit (100) includes a drive current generation circuit (120) for supplying a drive current in accordance with a data signal. The circuit (120) has a variable current source (122) having one terminal connected to a high potential side and supplying a reference current (I) from the other terminal thereof, and a switching transistor (124) for receiving the reference current (I) and generating a drive current in accordance with the data signal (DATA). The driver circuit (100) further includes a current mirror circuit (140) for conducting through a second node (134) a current having a magnitude of the current flowing through a first node (132) multiplied by a predetermined current mirror ratio. The current mirror circuit (140) has a first transistor (142) connected to the first node (132) and a second transistor (144) having a control terminal connected to the first node (132) and a control terminal of the first transistor (142), said second transistor (144) connected to the second node (134). To the second node (134) is connected a laser diode element (150) to be controlled.
In operation, the driver circuit (100) controls the current (Id) flowing through the laser diode element (150), while receiving the data signal (DATA). According to the data signal (DATA) that is a binary signal, the switching transistor (124) becomes either conductive or nonconductive. First, when the switching transistor (124) is nonconductive, no current flows through the first node (132), so that the first transistor (142) remains off. Thus, the second transistor (144) is also off, and no current flows through the second node (134) and laser diode (150). Next, when the switching transistor (124) is conductive, the reference current (I) from the variable current source (122) flows through the first node (132). Due to the nature of the current mirror circuit (140), a current (mI) having a magnitude of the reference current (I) flowing through the first node (132), multiplied by a predetermined current mirror ratio (m), flows through the laser diode (150) via the second node (134). In this way, the emitting and non-emitting states of the laser diode (150) can be switched in accordance with the binary signal (DATA).
Meanwhile, the optical output of the laser diode tends to vary significantly depending upon the operating temperature. For example, the light-emitting efficiency of the laser diode degrades as the temperature rises, so that the current conducted through the laser diode needs to be increased in order to maintain the optical output constant. Thus, to achieve a constant optical output regardless of the operating temperature, it is necessary to adjust the current (mI) conducted through the laser diode as appropriate depending upon the operating temperature. That is, it is necessary to adjust the magnitude of the reference current (I) as appropriate depending upon the operating temperature. Furthermore, in order to accommodate variability in product characteristics for laser diode elements on a unit-by-unit basis, as well as changes over time, the magnitude of the reference current (I) may be adjusted. In this context, the current source (122) is a variable current source. Thus, the magnitude of the reference current (I) is modified depending upon various factors.
However, if the reference current (I) is too small, a problem may occur. If the reference current (I) is small, the amplitude of the drive current also becomes small, so that the rise characteristics of the mirrored version (mI) of the drive current, that is, the current (Id) flowing through the laser diode (150), are degraded. Thus, there is a problem in that high-speed switching of the laser diode is difficult to achieve.